1. Field of the Invention
The present invention relates to an ejection head and an image forming apparatus, and more particularly to a structure of an ejection head which ejects a liquid onto an ejection receiving medium.
2. Description of the Related Art
In recent years, inkjet printers have come to be used widely as data output apparatuses for outputting images, documents, or the like. By driving recording elements, such as nozzles provided in a recording head, in accordance with data, an inkjet printer is able to form data onto a recording medium such as recording paper, by means of ink ejected from the nozzles.
In an inkjet printer, a desired image is formed on a recording medium by causing a print head having a plurality of nozzles and a recording medium to move relatively to each other, while causing ink droplets to be ejected from the nozzles.
The method for causing the ink to be ejected from the print head provided in an inkjet printer may be a piezoelectric method in which actuators, such as piezoelectric elements, are provided on a diaphragm (pressure plate) which forms a wall of pressure chambers in which the ink is accommodated, the pressure chambers are caused to deform by driving the actuators, and a volume of ink corresponding to the amount of change in the volume of the pressure chambers is ejected, or it may be a thermal method in which ink inside pressure chambers is heated by heaters provided in the pressure chambers, and ink is ejected due to the pressure of the bubbles generated inside the pressure chambers when the ink is heated.
In the inkjet printer, in order to achieve high quality in the image formed on the recording medium, a print head having a high (stable) ejection characteristic is required. In a print head using a piezoelectric method, the ejection characteristics of the print head can be improved by raising the conversion efficiency of the actuators by adopting various designs in the structure and shape of the actuators or the pressure chambers accommodating the ink.
The inkjet head disclosed in Japanese Patent Application Publication No. 5-246024 adopts a structure in which a thin plate of electrostrictive material, which is difficult to machine, has a single-plate structure and is machined to a high level of accuracy by plating, etching, or the like, and an elastic thin plate divided into two parts is provided. Thereby, it is possible to reduce the cost of the apparatus as well as achieving high-quality printing.
In the inkjet head disclosed in Japanese Patent Application Publication No. 7-290705, an elastic member is used for wall members forming pressure chambers between a nozzle plate and a pressure plate, thereby increasing the efficiency of conversion from the force generated by the piezo element to the ink ejection force, and furthermore, increasing the durability of the head (improving the resistance to stress acting from the diaphragm to the pressure chambers), facilitating assembly (improving the margin of tolerance relating to positional divergence), and preventing satellite effects (namely, suppressing residual vibrations).
The inkjet head and inkjet printer disclosed in Japanese Patent Application Publication No. 2001-26106 have a structure in which the functional distortion index of a piezoelectric element is taken to be 550×10−6 or more; the stress index is taken to be 30×106 or more; the electrical field intensity 3.0×106 or more; the width of the pressure chambers is taken to be no less than 220 μm and no more than 300 μm; the thickness bonding the piezoelectric element and the diaphragm is taken to be no less than 1.5 μm and no more than 30 μm; and the width of the pressure chambers is taken to be no less than 28 μm and no more than 330 μm. Thereby, it is possible to achieve printing characteristics providing good efficiency and satisfactory practicability.
The inkjet print head and piezoelectric converter disclosed in Japanese Patent Application Publication No. 2003-25573 have a structure in which the outer perimeter of a piezoelectric converter overlaps with the opening of a chamber. Thereby, the processing capacity of the piezoelectric converter is increased, the piezoelectric sensitivity is reduced, and manufacturing variation is also reduced.
In the liquid droplet ejection head, the ink cartridge, and the inkjet recording apparatus disclosed in Japanese Patent Application Publication No. 2004-66652, a diaphragm has a hinge structure forming a structural portion of partially reduced rigidity. Thereby, it is possible to achieve a high-density liquid ejection head which is little variations in ejection characteristics.
In the inkjet recording head and method for manufacturing same described in Japanese Patent Application Publication No. 2004-74806, upper electrodes and piezoelectric thin films are etched simultaneously. Thereby, since there is no divergence in pattern between the piezoelectric thin films and the electrodes, the electrical field is applied efficiently to the piezoelectric thin films so that satisfactory displacement can be obtained.
However, in a print head formed to a high density, the structure and shape of the actuators and pressure chambers are subject to dimensional restrictions, and these restrictions are even more severe in the case of a print head formed to particularly high density. In addition, when using a functional ink imparted with functional properties, such as smear preventing characteristics or fixing promoting characteristics, functional ink of this kind has higher viscosity than generally used inks, thereby requiring even better ejection characteristics.
In the inkjet head disclosed in Japanese Patent Application Publication No. 5-246024, since the electrostrictive thin plate makes contact directly with the ink, insulation processing should be applied to the electrostrictive thin plate. In addition, an electrostrictive thin plate disclosed in Japanese Patent Application Publication No. 5-246024 is difficult to manufacture. Furthermore, in Japanese Patent Application Publication No. 5-246024, there is no specific disclosure relating to regarding the displacement volume of the pressure chambers and the pressure generated by the electrostrictive thin plates, when the inkjet head is formed to a high density in order to achieve high-resolution printing.
The inkjet head disclosed in Japanese Patent Application Publication No. 7-290705 has a composition in which the volume of the pressure chambers is changed by deforming the wall members of the pressure chambers. Therefore, it is necessary for suppression of mutual interference between neighboring pressure chambers.
In the inkjet head and the inkjet printer disclosed in Japanese Patent Application Publication No. 2001-26106, the general shape of the print head comprising the pressure chambers, piezoelectric bodies, diaphragm, and the like, is stipulated. However, it is doubtful whether ejection characteristics can be made more efficient. In addition, there is no particular disclosure regarding variation within the print head.
In the inkjet-type print head and piezoelectric converter disclosed in Japanese Patent Application Publication No. 2003-25573, the object thereof is to suppress variation in the manufacture and assembly of piezoelectric converters, but there is no disclosure of technology for improving ejection characteristics. Additionally, when the amount of overlap between the chamber openings and the piezoelectric converters exceeds 5%, the volumetric displacement of the chambers declines, and therefore, it is necessary to install the piezoelectric converters on the chambers to a very high degree of accuracy.
In the liquid droplet ejection head, ink cartridge and inkjet recording apparatus disclosed in Japanese Patent Application Publication No. 2004-66652, when manufacturing the diaphragm having a hinge structure, the manufacturing process increases in complexity. In addition, variations in the quality of the manufactured hinge structure may have an influence on the ejection characteristics.
In the inkjet recording head and method of manufacturing same disclosed in Japanese Patent Application Publication No. 2004-74806, since the piezoelectric thin films and the electrodes are formed in the same shape, it is possible to obtain an effect in maximizing the pressure generated by the piezoelectric bodies. However, it is doubtful whether satisfactory displacement could be obtained when the viscosity (or other properties) of the ink to be ejected is changed.